Quick Links

How would you like to share?

Poor traffic control causes nasty pile-ups—not only for cars but for proteins, too. This realization fuels the study of intracellular pathways that regulate the comings and goings of amyloid-β precursor protein (APP). Three recent papers solidify the role of the retromer, a protein complex mediating endosome-to-Golgi transport, in APP processing and in Alzheimer’s disease. They help explain how the Aβ production engine runs hot when APP gets stuck in endosomal traffic jams.

In one study, described in the June 4 Neurobiology of Aging online, researchers led by Lindsay Farrer of Boston University School of Medicine, Massachusetts, and Matthew Seaman of the University of Cambridge, U.K., identified AD-associated variants in several genes that regulate retromer function. Another study found that the neuronal retromer regulates APP transport primarily in dendrites and axons, and that retromer deficiency leads to APP buildup and Aβ overproduction. That work, published online April 6 in Neurobiology of Disease, was led by Scott Small and Gilbert Di Paolo of Columbia University in New York. And in the January 25 Journal of Neuroscience, Olav Andersen of Aarhus University, Denmark, and colleagues identify a motif in the neuronal sorting receptor SorLA that binds retromer and regulates APP processing. These reports “consolidate the idea that abnormal intracellular trafficking of APP by SorLA is an underlying cause of amyloidogenic processing,” noted Thomas Willnow of Max Delbrück Center for Molecular Medicine, Berlin, Germany, in an e-mail to Alzforum. Willnow is a coauthor on the Journal of Neuroscience paper.

Farrer and Seaman build on prior work that identified SorLA (aka SORL1) as a risk gene for late-onset AD (see ARF related news story on Rogaeva et al., 2007) and that suggested the receptor influences AD pathology by directing APP compartmentalization (see ARF related news story on Schmidt et al., 2007). Seaman's lab identified new retromer or retromer-associated genes via an siRNA library screen. First author Badri Vardarajan and colleagues in Farrer’s group then tested 15 of the top hits for association with AD in the large Caucasian dataset (8,309 AD cases and 7,366 normal elderly) provided by the Alzheimer’s Disease Genetics Consortium (ADGC), which identified new AD risk genes last year (see ARF related news story on Naj et al., 2011). Four of the new retromer genes associated with AD—RAB7A, KIAA1033, SNX1, and SNX3. The KIAA1033 association replicated in a smaller African-American cohort, as did SNX3 variants in an Israeli-Arab dataset. The findings emphasize the importance of protein trafficking to AD pathology.

Seaman’s lab further characterized SNX3, the gene showing the most robust association with AD, and RAB7A, a GTPase that helps recruit the retromer into endosomes (see Seaman et al., 2009). Using yeast two-hybrid assays and transfected HeLa cells, the researchers demonstrated that RAB7A and SNX3 interact with the retromer through independent mechanisms to regulate how the complex interacts with cell membranes. This paper “achieves two things,” said Small. “First, it adds to previous genetics findings showing that retromer dysfunction plays a pathogenic role in AD. Second, it uses cell biological techniques to expand our understanding of retromer biology.”

A previous microarray analysis of brain tissue samples by Small and colleagues implicated the retromer complex in AD by showing reduced levels of retromer proteins Vps35 and Vps26 in the entorhinal cortex of AD patients (Small et al., 2005). These brain areas are especially vulnerable to the disease. The researchers predicted that SorLA mediates retromer trafficking of APP, and that retromer dysfunction would cause APP to cluster inappropriately in endosomes, where levels of β-secretase are high. That could drive production of Aβ. “Our new findings confirm the second prediction,” Small told Alzforum.

As they reported in Neurobiology of Disease, Small, Di Paolo, and colleagues clarified what role the retromer plays in neurons. Most retromer cell biology studies to date were based on non-neuronal cells, Small said. Imaging mouse hippocampal neurons, first author Akhil Bhalla and colleagues detected Vps35 in endosomes and the Golgi, where the retromer typically functions. They also found Vps35 in dendrites and axons, suggesting it might be involved in long-range transport, including, perhaps, trafficking of APP. Sure enough, when they silenced Vps35 in hippocampal neurons using lentiviral shRNA, APP accumulated in early endosomes in processes, but not in the soma. Furthermore, APP colocalized more with BACE1, and Aβ levels rose. “The real novelty of this paper is that it shows that the retromer's involvement in APP trafficking occurs mostly in distal processes,” Small said. “Retromer dysfunction appears to ‘load up’ APP in endosomes of dendrites and axons—sites where APP processing is most likely to proceed.”

Olav Andersen’s work reported in the Journal of Neuroscience paper fleshes out the retromer’s role in APP processing even further by showing how the protein complex interacts directly with SorLA. Using biochemistry, cell biology, and electron microscopy approaches, first author Anja Fjorback and colleagues show that the retromer component that binds to SorLA is Vps26. More specifically, Vps26 binds a hexapeptide motif (FANSHY) in the cytoplasmic tail of the sorting receptor. SorLA mutants lacking this motif still bound APP, but failed to direct the precursor to the Golgi, leaving it more susceptible to amyloidogenic processing. Again, this work reveals the vital role played by the retromer and by SorLA to control Aβ production.

The retromer might play a similar role in other diseases as well. Recently, research linked Vps35 mutations to late-onset Parkinson’s disease (see ARF related news story). Based on previous work, it was assumed that Vps35, not Vps26, bound SorLA, Small said. The new data are important because “they might explain how a single complex, the retromer, could be involved in different diseases,” said Small. He speculated that maybe Vps35 is linked to receptors that are important in PD, whereas by virtue of its binding SorLA, Vps26 is more linked to APP and AD.—Esther Landhuis

Comments

Comments on this content

Faulty transport along the endocytic route in neurons is emerging as an important molecular mechanism underlying enhanced APP processing in AD. One pathway elucidated in some detail entails SorLA (aka SORL1 or LR11), a neuronal sorting protein for APP, and retromer, a trafficking adaptor complex that sorts cargo from endosomes to the Golgi. Previously, a number of studies provided independent experimental evidence implicating impaired expression of SORLA and retromer in aggravated APP processing and amyloid-β peptide production in both animal models and in patients. From these studies, a model was proposed whereby SorLA re-routes internalized APP molecules from early endosomes back to the Golgi, bypassing delivery of the precursor protein to late endosomes where β-secretases reside. Because the cytoplasmic tail of SorLA includes a proposed binding motif for retromer, this adaptor complex was suggested to direct retrograde trafficking of SorLA/APP complexes from endosomal to Golgi compartments.

Now, three studies have further substantiated this model by providing important additional evidence to support a role for retromer in SorLA-dependent sorting of APP. Thus, work by Fjorback et al. finally confirmed a hexapeptide motif (FANSHY) in the cytoplasmic tail of SorLA as a binding site for Vps26, a subunit of the retromer complex. A SorLA mutant lacking the FANSHY motif retained APP-binding activity but failed to properly directe APP to the Golgi, resulting in increased amyloidogenic processing. In a study from the lab of Scott Small, immunocytochemical investigations were applied to elucidate, in detail, the trafficking routes taken by retromer complex in primary neurons. Gratifyingly, these studies confirmed the necessity of retromer to guide long-range transport of APP along the axonal path. Knockdown of Vps35, another subunit of the retromer complex, resulted in accumulation of APP in endosomal compartments, in increased colocalization with BACE, and in elevated levels of Aβ.

Finally, a new study published by Matthew Seaman and Lindsay Farrer and their colleagues identified genetic association of AD with several vesicular trafficking proteins. Amongst these, SNX3 and RAB7A were further shown to represent novel regulatory components to control membrane association of retromer. Taken together, these recent studies provide exciting new data that consolidate abnormal intracellular trafficking of APP by SORLA as an underlying cause of amyloidogenic processing. It will be exciting to learn more about the molecular mechanisms whereby sequence variations affect expression or activity of pathway components in individuals at risk of sporadic AD.

The three recent papers discussed here (1-3) shed new light on the role of retromer in intracellular trafficking, and on the proteolytic processing of the amyloid-β precursor protein (APP) and the consequences of its abnormal function for the pathogenic process in Alzheimer’s disease (AD). Retromer is an adaptor protein with roles in regulating the trafficking between endosomes and the Golgi apparatus, most likely retrograde trafficking. Other adaptor proteins that regulate various steps along the complex route of APP transport to and from the cell surface, and between intracellular compartments, could similarly impact the processing of APP. This is the case with Fe65 (4), Mint1/X11 (5), JIP-1 (6,7), and DISC1 (8), to name just a few of them. Thus, it becomes clear that the aberrant processing of APP that leads to increased generation and/or decreased clearance of Aβ is likely caused by diversion of APP from its normal transport route. Accordingly, searching for proteins that perturb trafficking of APP using large-scale screening assays is now more important than ever. Using dual immunolocalization cytochemistry, we have previously found that, in neurons, Aβ accumulations within neurites strongly colocalize with BACE1 and Rab7 (9), a small GTPase that regulates late endocytic trafficking, including recruitment of retromer to endosomes. Interestingly, Vardarajan et al. (1) identified significant association of AD with SNPs in the Rab7A gene. However, we note that Rab7, while regulating late endosomal trafficking, is also required for the normal progression of autophagy (10), another trafficking pathway proposed to be dysregulated in AD. Since it is known that the retromer also regulates autophagocytosis (11), one wonders whether abnormal function of the retromer affects the generation of Aβ along the endocytic or autophagocytic pathway.

We would also like to draw attention to another issue covered by these interesting papers that is not fully settled—the site of action of the retromer. This has major implications for another unsolved problem—the intracellular site of production and accumulation of Aβ. Currently, it is accepted that the retromer plays a role in regulating the endosome-to-Golgi retrieval pathway. Much of this retrieval does take place in the cell body, as many previous studies have shown, and a significant fraction of cell surface APP is internalized, and Aβ is generated in endosomes in the neuronal soma (see, e.g., our studies [8,9]). However, Bhalla et al. (2) now show that the retromer may primarily function in axons and dendrites rather than in the soma, an interesting finding that needs to be confirmed in future studies. Since Aβ is also generated at the synapse (12), would abnormal retromer function facilitate generation and accumulation of Aβ at synapses?

Fjorback et al. (3) clarify the mechanism by which the retromer regulates the processing of APP, and show a direct interaction of the retromer with SorLA, a sorting receptor for APP, previously shown to be linked to AD. According to the proposed mechanism, the SorLA-retromer complex normally functions to retrieve APP via the endosome-to-Golgi pathway. This model nicely explains the increased generation of Aβ in an endosomal compartment when the SorLA-retromer complex does not properly function. Still, the precise site of action of SorLA, as well as the site of intracellular generation of Aβ, remain issues not fully understood (13). Certainly, there is still much to be learned from future studies about the relationship between trafficking and processing of APP, as well as about the relevance of abnormal intracellular transport of APP for AD (14).

See also:

Muresan, V. and Z. Muresan, DISC1 controls production of amyloid-β (Aβ) by regulating intracellular trafficking of the Aβ precursor protein (APP) along the secretory, endocytic, and degradative route. Annual Meeting of the Society for Neuroscience, Washington, D.C., November 12-16, 2011.